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New 80mm refractor

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#1 Cydonia76

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Posted 23 September 2021 - 02:38 AM

I’m looking at buying either the William Optics Zenithstar f6.9 81mm or the Sky Watcher Black diamond 80mm f7.5

 

the intended purpose is visual. I’m wanting one that is sharp and no field curvature with my Naglar 20mm and pan optics 27mm  eyepiece

 

my question is how much better corrected will the Skywatcher be over the WO since it has a slower focal ratio of 7.5.

 

I recently tried a 72mm f5.9 and it had plenty of field curvature with my eyepieces. I did not care for it much

 

Nick



#2 Far Star

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Posted 23 September 2021 - 05:16 AM

If I were you, I would also consider a Vixen SD81S. The optical quality of this Japanese-made refractor is outstanding and on the same level as the Takahashi Fluorite Doublets.


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#3 Cydonia76

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Posted 23 September 2021 - 06:33 AM

If I were you, I would also consider a Vixen SD81S. The optical quality of this Japanese-made refractor is outstanding and on the same level as the Takahashi Fluorite Doublets.

I would love the Vixen SD81S but it is a lot more expensive then the other two


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#4 Mihai

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Posted 23 September 2021 - 08:23 AM

If I were you, I would also consider a Vixen SD81S. The optical quality of this Japanese-made refractor is outstanding and on the same level as the Takahashi Fluorite Doublets.


That's a very bold statement,bold enough to start a war over here 😁
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#5 Alterf

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Posted 23 September 2021 - 09:01 AM

I find field curvature at less than 600mm of focal length annoying, so when I had the same decision, I opted for the longer focal length SW scope.  FWIW, I think with those eyepieces you will still see some field curvature in the SW, but it will be less than in the other. 

 

The chromatic correction may or may not be noticeable, depending on how sensitive you are to it.  I've owned 80mm ED scopes of both focal lengths (not WO's but another brand's), and I noticed the difference between them after I had used each for a while. 

 

For visual, I much prefer the SW.  I found the mechanics on the SW model not great, however.  I replaced the focuser and tube rings.

 

Val


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#6 Cydonia76

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Posted 23 September 2021 - 06:38 PM

Than

 

I find field curvature at less than 600mm of focal length annoying, so when I had the same decision, I opted for the longer focal length SW scope.  FWIW, I think with those eyepieces you will still see some field curvature in the SW, but it will be less than in the other. 

 

The chromatic correction may or may not be noticeable, depending on how sensitive you are to it.  I've owned 80mm ED scopes of both focal lengths (not WO's but another brand's), and I noticed the difference between them after I had used each for a while. 

 

For visual, I much prefer the SW.  I found the mechanics on the SW model not great, however.  I replaced the focuser and tube rings.

 

Val

Hi Val,

 

thanks for your reply, I found your information very handy. I really want to have a wide field with minimal field curvature to give me the space walk feel.


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#7 Jon Isaacs

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Posted 23 September 2021 - 08:28 PM

I’m looking at buying either the William Optics Zenithstar f6.9 81mm or the Sky Watcher Black diamond 80mm f7.5

 

the intended purpose is visual. I’m wanting one that is sharp and no field curvature with my Naglar 20mm and pan optics 27mm  eyepiece

 

my question is how much better corrected will the Skywatcher be over the WO since it has a slower focal ratio of 7.5.

 

I recently tried a 72mm f5.9 and it had plenty of field curvature with my eyepieces. I did not care for it much

 

Nick

Nick:

 

Both those scopes will have about the same amount of field curvature.  The F/7 will have slightly more but it will also have a slightly wider field of view.  Going to longer focal length telescopes is a catch 22 because if you want to keep the TFoV, you need to go to an eyepiece with a larger field stop.  

 

It is possible to get very wide fields of view that are free of field curvature.  The TSFLAT2 is a field flattener with sufficient backfocus to be used with a 2 inch diagonal.  Getting the spacing right is a little tricky but I use it with the 31mm Nagler and 80mm F/5 (6.0 degrees) and F/6 refractors (5.0 degrees) and the fields are in focus from the center to the edge.  

 

https://www.teleskop...-Anschluss.html

 

Jon


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#8 Cydonia76

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Posted 23 September 2021 - 08:48 PM

Nick:

 

Both those scopes will have about the same amount of field curvature.  The F/7 will have slightly more but it will also have a slightly wider field of view.  Going to longer focal length telescopes is a catch 22 because if you want to keep the TFoV, you need to go to an eyepiece with a larger field stop.  

 

It is possible to get very wide fields of view that are free of field curvature.  The TSFLAT2 is a field flattener with sufficient backfocus to be used with a 2 inch diagonal.  Getting the spacing right is a little tricky but I use it with the 31mm Nagler and 80mm F/5 (6.0 degrees) and F/6 refractors (5.0 degrees) and the fields are in focus from the center to the edge.  

 

https://www.teleskop...-Anschluss.html

 

Jon

Hi Jon,

 

thanks for your reply, would you happen to know how much of the field will be noticeably affected by field curvature in percentage terms?
 

if I go with the SW ED80 I will need to replace the focuser where as the Wo ZS81 I will not. I like your suggestion of the TS flattener. Does that simply screw into the barrel of the 2” diagonal? And your certain I will have enough back focuse with my eyepieces?
 

cheers



#9 Jon Isaacs

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Posted 23 September 2021 - 09:04 PM

Hi Jon,

 

thanks for your reply, would you happen to know how much of the field will be noticeably affected by field curvature in percentage terms?
 

if I go with the SW ED80 I will need to replace the focuser where as the Wo ZS81 I will not. I like your suggestion of the TS flattener. Does that simply screw into the barrel of the 2” diagonal? And your certain I will have enough back focuse with my eyepieces?
 

cheers

 

- I can't really give you a number on the percentage of the field that will be affected by field curvature, it depends in part on your eyes and your perception. Someone who was used to using coma corrected Dobs and flat field TeleVue refractors might be more sensitive than someone else.  

 

- Properly adjusted, the Skywatcher two speed focuser is plenty good enough for visual. 

 

- The TSFLAT2 doesn't quite screw on to the nosepiece of a 2 inch diagonal.  That ends up being somewhat too much.  What I did was use a diagonal with a nose piece threads on using the standard 48mm threads.  I removed the nose piece, threaded the TSFLAT2 in it's place so the TSFLAT2 is the nosepiece.

 

Finding that diagonal is something of a challenge.

 

Jon


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#10 Cydonia76

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Posted 24 September 2021 - 05:15 AM

- I can't really give you a number on the percentage of the field that will be affected by field curvature, it depends in part on your eyes and your perception. Someone who was used to using coma corrected Dobs and flat field TeleVue refractors might be more sensitive than someone else.  

 

- Properly adjusted, the Skywatcher two speed focuser is plenty good enough for visual. 

 

- The TSFLAT2 doesn't quite screw on to the nosepiece of a 2 inch diagonal.  That ends up being somewhat too much.  What I did was use a diagonal with a nose piece threads on using the standard 48mm threads.  I removed the nose piece, threaded the TSFLAT2 in it's place so the TSFLAT2 is the nosepiece.

 

Finding that diagonal is something of a challenge.

 

Jon

Do you know if it will screw into a WO 2” diagonal, in replace of the 2” nose piece?



#11 Jon Isaacs

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Posted 24 September 2021 - 05:18 AM

Do you know if it will screw into a WO 2” diagonal, in replace of the 2” nose piece?

 

I don't know about specific WO 2 inch diagonals but the diagonal I am using is a 2 inch William Optics carbon fiber dielectric diagonal.  

 

If you have a 2 inch (48mm thread) filter, if you can unscrew the nosepiece on your diagonal, try screwing the filter to the diagonal.  That's the same thread the TSFLAT2 uses.

 

Jon


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#12 grif 678

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Posted 25 September 2021 - 03:11 PM

Vixen has discontinued making the SD81f, was looking yesterday



#13 Cydonia76

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Posted 25 September 2021 - 03:14 PM

Thanks Jon, I did check mine and it will screw into the diagonal.

 

after more research I’m worried the WO ZS81 will suffer the “iron cross” problem and I have read the WO scope have a soft focus to them

 

them:SW ED80 has nothing but good reviews optically



#14 Mihai

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Posted 25 September 2021 - 03:19 PM

Vixen has discontinued making the SD81f, was looking yesterday


Is this the same scope as Vixen SD81S ?

#15 SilverLitz

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Posted 25 September 2021 - 06:02 PM

I believe the scopes with an "f" at the end are Chinese made, the others, without the "f", are made by Vixen in Japan and are of higher quality.



#16 Sky Muse

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Posted 25 September 2021 - 06:20 PM

There is no SD81f, only the SD81S, made in Japan, and as it ought to be.  Bill P. likes his a lot, and it's probably the short-tube 80mm I'd choose, in the end.



#17 Cydonia76

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Posted 27 September 2021 - 04:30 PM

- I can't really give you a number on the percentage of the field that will be affected by field curvature, it depends in part on your eyes and your perception. Someone who was used to using coma corrected Dobs and flat field TeleVue refractors might be more sensitive than someone else.  

 

- Properly adjusted, the Skywatcher two speed focuser is plenty good enough for visual. 

 

- The TSFLAT2 doesn't quite screw on to the nosepiece of a 2 inch diagonal.  That ends up being somewhat too much.  What I did was use a diagonal with a nose piece threads on using the standard 48mm threads.  I removed the nose piece, threaded the TSFLAT2 in it's place so the TSFLAT2 is the nosepiece.

 

Finding that diagonal is something of a challenge.

 

Jon

Hi Jon,

 

after a lot of mulling on which to get I think I will go with the ZS81 and do your suggestion of using the TSFLAT2 on my diagonal to give the flat field. I’m sure the SW ED80 is a good scope for the money, but the mechanics of the ZS81 is appealing.

 

You mentioned that getting the spacing was a bit tricky, what do you mean by that, is it not just a matter of screwing the TSFLAT2 into the diagonal and off I go?
 



#18 Jon Isaacs

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Posted 27 September 2021 - 10:25 PM

You mentioned that getting the spacing was a bit tricky, what do you mean by that, is it not just a matter of screwing the TSFLAT2 into the diagonal and off I go?

 

It might be that simple.  it worked for me.  

 

Jon



#19 mrsjeff

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Posted 28 September 2021 - 04:52 PM

There is no SD81f, only the SD81S, made in Japan, and as it ought to be.  Bill P. likes his a lot, and it's probably the short-tube 80mm I'd choose, in the end.

Yes, the Synta scope sold by Vixen is the ED80sf. It's akin to the Sky-Watcher ED80 and the Orion 80ED. As noted by the OP, many many people very much appreciate the views offered by these Synta scopes. Not a bad choice at all. 




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